1. Field of the Invention
This invention relates to polyvinyl alcohol hydrogels. More particularly, this invention relates to polyvinyl alcohol hydrogels containing uniformly dispersed and highly oriented crystalline regions that substantially improve the physical properties of the hydrogel.
2. Background of the Invention
The chemical and physical properties of polyvinyl alcohol hydrogels make them useful materials for a variety of end use applications, including filtration and ion exchange membranes, bio-compatible films, fibers and other shaped articles, contact lenses, devices for the controlled delivery of materials, coatings and filler materials.
The formation and physical properties of polyvinyl alcohol hydrogels are discussed in a review article by Hyon in the journal "Kobunshi Kako" 39(6) 304-10 (1990) [CAl14(16):144304x].
Gels, fibers and other products prepared from solutions of polyvinyl alcohol in water or a mixture of water and a water-miscible organic solvent such as methanol, propanol, dimethyl sulfoxide and N,N-dimethylformamide are well known from numerous patents and other publications. Typical of recently issued patents describing gels formed from aqueous solutions of polyvinyl alcohol is U.S. Pat. No. 4,663,358, which issued to Hyon et al on May 5, 1987 and U.S. Pat. No. 4,851,168, which issued to D. Graiver et al. on Jul. 25, 1989. These patents teach cooling to below room temperature a solution of polyvinyl alcohol in a mixture of water and a water-miscible organic solvent such as alcohols containing from 1 to 4 carbon atoms, glycols and dimethyl sulfoxide. Mixtures of water and dimethyl sulfoxide are preferred, and the water can constitute from 10 to 90 weight percent of the solvent mixture. The gel formed by cooling the solution Is then immersed in flowing water to remove the organic solvent.
Hydrogels prepared as described in the Hyon and Graiver et al. patents are transparent whereas gels formed using a solution of polyvinyl alcohol in either water or dimethyl suifoxide as the only solvent are opaque.
For some end uses such as biomedical applications it is desirable to maximize the tensile strength and other physical properties of polyvinyl alcohol hydrogels. This is particularly true for hydrogels containing more than about 40 weight percent of water.
Techniques that have been used to improve the physical properties of polyvinyl alcohol hydrogels typically involve crosslinking by radiation or chemical means, increasing the crystallinity of the polymer and/or adding reinforcing agents. The improvements in tensile properties achieved by crosslinking are typically less than desirable, and are accompanied by reduced swelling in water, which is desirable for some applications, and insolubility of the polymer in water and organic solvents such as hot water and dimethyl suifoxide that dissolve non-crosslinked polymers.
The aforementioned U.S. Pat. No. 4,851,168, issued to D. Graiver et al. and Japanese patent publication no. 1/257,026, published on Oct. 13, 1989 teaches preparing high strength fibers and films from polyvinyl alcohol (PVA) hydrogels by spinning or extruding solutions of PVA with degrees of polymerization greater than 1500 and drawing the resultant products using draw ratios greater than 10. The Japanese patent publication teaches shrinking the resultant fiber or film in a water bath at a temperature at least 5 degrees below but not more than 50 degrees below the temperature at which the PVA will dissolve in the bath.
Hyon et al. in the journal "Kobunshi Ronbunshu" 46 (11) 673-80 (1989) [CA112(12):99763s] teach freezing a concentrated aqueous solution of PVA followed by slow crystallization of the frozen polymer above its freezing point to produce a semi-crystalline polymer with a microporous structure.
The use of repeated freeze-thaw cycles to increase the strength and rigidity of PVA hydrogels intended to control the release of pharmaceuticals and serum albumin is taught by N. Peppas et al. [J. Controlled release, 18(2), 95-100; CA116(16): 158711f]
The preparation of microspheres of non-crosslinked PVA exhibiting good compressive strength by dispersing an aqueous solution of the polymer in a water-immiscible liquid, freezing the solution and crystallizing the PVA at a temperature of 0 to 10.degree. C. is reported in Japanese patent publication 62/45637, published on Feb. 22, 1987.
The preparation of semicrystalline hydrogels from copolymers derived from vinyl trifluoroacetate and comonomers such as maleic acid is described by R. Ofstead et al. in Adv. Chem. Ser., 223 (Polym. Aqueous Media) , 61-72 [CA112(6):42493z]
The preparation of reinforced composites by blending polyvinyl alcohol, water and alumina, compressing the resultant mixture and aging it with heating is described by Sakai et al. in Japanese patent publication no. 1/236,274. An exemplified composite exhibited a flexural strength of 40 MPa.
PVA/silica composite hydrogels are reported by S. Ikoma et al. in "Kobunshi Ronbunshu" 47(12), 1001-4 (1990) [CA114(8) : 63302t). The improved mechanical and theological properties achieved by the addition of silica are attributed to microscopic bond formation between the silica and the PVA.
It is known in the art relating to reinforcement that for a reinforcing agent to function effectively it must be bonded to the matrix material. In the past this type of bonding has typically been achieved using coupling agents such as silanes containing organofunctional groups.
One objective of this invention is to improve the physical properties such as tensile strength, modulus and toughness of non-crosslinked polyvinyl alcohol hydrogels by providing uniformly dispersed highly crystalline regions that are bonded to the adjoining polymer matrix.
A second objective of this invention is to provide a method for introducing highly crystalline regions into a polyvinyl alcohol hydrogel. The regions function as reinforcing agents for the hydrogel.